I am running an A Spec rear right now. Mounted it up a week or two ago. I really couldn't tell a difference other that maybe a stiffer sidewall. They claim it is an extra ply. My bike is 640 wet weight, so i figured I should try the A Spec.
My VFR12 is right around 600lbs. I run an A spec rear and a regular front. When I bought it it had a new set of regular GTs mounted.
I run the A spec with 2 less psi and it feels pretty much the same to me...ymmv